1. Field of the Invention
This invention relates to an isolated operation prevention device for a distributed power supply in a power system, which monitors and detects, at the customer equipment side, the interruption of the system supply due the opening of a circuit breaker at a substation and disconnects the distributed power supply from the system when the system supply is stopped to prevent isolated operation of the distributed power supply, and, to be more detailed, concerns the compensation of power fluctuations when a wind power generator, etc. is used as the distributed power supply.
2. Description of the Related Art
Conventionally, with customer equipment in factories, large-scale buildings, etc., so-called distributed power supplies have been interconnected with the power system, and shortages of the power generated by the distributed power supplies have been compensated by the system power.
In such cases, when a circuit breaker at a substation of a power company is opened due to a system fault, etc. and the system supply is stopped, the distributed power supply must be disconnected from the system and prevented from performing isolated operation in order to prevent the occurrence of electric shock accidents, etc. due to isolated operation of the distributed power supply.
As means for preventing this isolated operation without fail, the present applicant has invented, as disclosed in Japanese Patent Unexamined Publication No. Hei 10-248168 and Japanese Patent Unexamined Publication No. Hei 11-252806, isolated operation prevention devices for distributed power supply that inject an intermediate-order harmonic current, which is synchronized with the fundamental and has a frequency that is a non-integer multiple of the fundamental, into the system and disconnect the distributed power supply upon detecting the stoppage of the system supply from a change of the impedance or admittance for the injected frequency at the upstream side of the system.
Japanese Patent Unexamined Publication No. Hei 10-248168 discloses the detection of the stoppage of the system supply from a change of the amount (magnitude) of impedance or admittance itself, and Japanese Patent Unexamined Publication No. Hei 11-252806 discloses the detection of stoppage of the system supply based on a change of impedance or admittance that is equal to or a greater than a fixed value in the capacitive direction.
The above-described isolated operation prevention devices for distributed power supply are equipped only with the function of disconnecting the distributed power supply upon detection of the stoppage of the system supply.
Meanwhile, with the above-described type of customer equipment, when the voltage generated by the distributed power supply fluctuates during interconnected operation, the system voltage fluctuates due to the active and reactive power fluctuations of the distributed power supply, and especially in the case where the distributed power supply is comprised of a wind power generator, etc. with a large capacity, the flicker fluctuation (ripple fluctuation), due for example to the moment-to-moment wind power fluctuation that is overlapped onto the gradual voltage variation due to average variations of the wind power, is large and the fluctuation of the system voltage due to this fluctuation becomes a problem.
Priorly in order to restrict this fluctuation of the system voltage, a separate power compensation device was equipped in addition to the isolated operation prevention device for distributed power supply.
This power compensation device is formed, for example, in the same manner as a thyristor controlled reactor (TCR) system or an inverter (self-excited) type static VAR compensator (SVC) such as described in pp. 28-31 of the Nisshin Electric Technical Report Vol. 41, No. 3 (""96, 11) and compensates for the voltage fluctuation by phase control by the switching of a reactor or condenser or by the injection of an active or reactive compensation power by drive control of an inverter device.
Thus especially in the case of a receiving equipment equipped with a distributed power supply comprised of a wind power generator, etc., which has a large capacity and with which the generated power fluctuates, there is a need to equip a power compensation device in addition to the isolated operation prevention device for distributed power supply, thus making it necessary to make the customer equipment large in scale and requiring vast equipment investments on the part of the customer.
Examples of conventional distributed power supplies for customers, etc., which are put to interconnected operation with a system and with which a power converter, such as an inverter, etc., is connected to the system, include micro gas turbine generation systems, wind power generation systems, solar power generation systems, fuel cell systems, emergency power supply systems that use a generator, flywheel, etc., UPS, and constant-use power supply systems (cogeneration equipment).
When the system is normal, these distributed power supplies generate interconnected operation power that is synchronized with the system fundamental by means of an inverter or other power converter (power inverter) and supplies this power to the system.
When the circuit breaker of a substation is opened and the system supply is stopped, the isolated operation of the distributed power supply is detected and the distributed power supply is disconnected from the systems to prevent the occurrence of electric shock accidents, etc. due to isolated operation.
As a related art of the abovementioned isolated operation detection system, active systems described for example in pp. 24 to 25 of the literature, xe2x80x9cDescription of Technical Requirement Guidelines for Power System Interconnection ""98xe2x80x9d (3rd. edition, Denryoku Shinposha Co., Ltd., Sep. 24, 1998) are known.
Active systems can be classified largely into reactive power fluctuation systems (xcex94Q systems) and active power fluctuation systems (xcex94P systems).
(i) Reactive power fluctuation system (xcex94Q system)
With this system, a periodic reactive power fluctuation is added to the generated output and the periodic voltage fluctuation or current fluctuation, etc. that appears upon transition to isolated operation is detected.
(ii) Active power fluctuation system (xcex94P system)
With this system, a periodic active power fluctuation is added to the generated output and the periodic frequency fluctuation or voltage fluctuation, etc. that appears upon transition to isolated operation is detected.
As is clear for example from FIG. 5(a) of p.25 of the above-mentioned literature, in the case of a distributed power supply, with which isolated operation is detected by a conventional active detection system, since a reactive power fluctuation or an active power fluctuation is caused by adding modulation to the system fundamental output of the power converter and the isolated operation that accompanies the stoppage of the system supply is detected from the fluctuation of the fluctuation detection signal (active signal), it has the problem of imposing ill effects, such as flicker (reactive power fluctuation) or beating (active power fluctuation), on the system.
Also as is described in the xe2x80x9c(1) Disconnection Time Intervalxe2x80x9d section in p. 152 of the above-mentioned literature, due to the fluctuation characteristics of the above-mentioned active signal, the conventional art requires approximately 3 to 10 seconds for detection of isolated operation. Time is thus required from the stoppage of the system supply to disconnection, and for example in the case where the reclosing time of a system that is one level above the system to which the distributed power supply is connected is extremely short and is less than 3 seconds, the disconnection cannot be performed in time to accommodate for the reclosing operation.
In the case where a plurality of customers using the same system have such types of distributed power supplies and a power equipment with which a plurality of distributed power supplies are connected is formed, when each of the distributed power supplies of this equipment detects isolated operation for example by the xcex94Q system, a circumstance may occur where the reactive power of a certain power supply fluctuates in the manner, +xcex94Q, xe2x88x92xcex94Q, +xcex94Q, xe2x88x92xcex94Q, . . . while the reactive power of another power supply fluctuates in the manner, xe2x88x92xcex94Q, +xcex94Q, xe2x88x92xcex94Q, +xcex94Q, . . . , and in this case, the power fluctuations of these power supplies will cancel each other out, making detection of isolated operation difficult.
Thus with such power equipment, there is a need to use some form of synchronization means to adjust the detection timings of the respective distributed power supplies.
Further, as a conventional art, the present applicant has disclosed in Japanese Unexamined Patent Publication No. Hei. 10-248168 (H02J 3/38), etc. inventions with which an interharmonic (intermediate-order harmonic), which is synchronized with the system fundamental and has a frequency that is a non-integer multiple of the system fundamental, is injected into a power system, the interharmonic is detected from measurement signals of the voltage and/or current of the system, and the stoppage of the system is detected based on this detection result to detect and stop the isolated operation of a customer""s distributed power supply upon stoppage due to a fault interruption, etc. of the power system.
In this case, since the interharmonic is of a frequency that does not exist inherently in the system, an advantage is provided in that the stoppage of the system can be detected by the supplying (injection) of an interharmonic of a small amount corresponding for example to approximately 0.1% of the system supply.
When, as described in the above-mentioned patent publication, a timing command, which has a sampling frequency that is PLL synchronized with the system voltage, is formed and the measurement signals of the voltage and current of the system are sampled based on this command, though the interharmonic of the injected order can be extracted and detected by subjecting the sampling data to a digital filter process based on a known Fourier transform at good precision without being affected by the system fundamental and harmonics having frequencies that are integer multiples of the fundamental system, in other words, the system components, a complex PLL circuit, etc. is required and the detection cannot be performed in an inexpensive and simple manner.
Meanwhile, if the PLL circuit for PLL synchronization is eliminated, a timing command for performing sampling in a non-synchronous manner with respect to the voltage of the power system (shall be referred to hereinafter as the system voltage) is formed, and the measured signals are sampled based on this command, though the arrangement will be lower in cost and simpler than the case where a PLL circuit is used, errors in the system voltage will arise in the extraction by Fourier transform, making detection of high precision difficult and disabling detection all together in some cases.
The same problems occur not only in the above-described case of detection of isolated operation of a distributed power supply but also when a fixed frequency sampling system is employed in cases where an interharmonic is injected into the system, the measurement signals of voltage and/or current of the system are sampled, and the interharmonic contained in the measured signals are extracted by a Fourier transform method to measure the harmonic characteristics of the system, etc.
A first object of this invention is to enable prevention of isolated operation of a distributed power supply and power fluctuation compensation (flicker fluctuation compensation) in the same manner as in the conventional art while enabling such types of customer equipment to be made small in scale and equipment investments by the customer to be lessened.
In order to achieve the above object, a first embodiment of this invention provides an isolated operation prevention device for distributed power supply, which is equipped with a power fluctuation monitoring unit, which monitors and detects the power fluctuation of the flicker fluctuation, which accompanies the fluctuation of wind power, etc., at the receiving point from the measurement results of the voltage at the intermediate-order harmonic current injection point and the current in the incoming line,
a power compensation unit, which, based on the power fluctuation detection result, forms a compensation power injection signal for canceling out the power fluctuation, and
an inverter device, which is driven and controlled by a signal obtained by adding the compensation power injection signal to the intermediate-order harmonic current injection signal and injects the intermediate-order harmonic current and the compensation power into the injection point.
The flicker fluctuation of the power at the receiving point, which is based on fluctuation of the power generated by the distributed power supply comprised of a wind power generator, etc., is thus detected by the power fluctuation monitoring unit, and based on this detection, the power compensation unit forms a compensation power injection signal that cancels out the power fluctuation.
The driving of the inverter device is then controlled by the signal obtained by adding the intermediate-order harmonic current injection signal and the compensation power injection signal, and this inverter device is used for the injection of the intermediate-order harmonic current and the injection of the compensation power, which cancels out the power fluctuation that is based on the fluctuation of the power generated by the distributed power supply.
A power fluctuation compensation function, which makes use of the inverter device, etc., is thus added to an isolated operation prevention device for distributed power supply to form a receiving equipment that is eliminated of the solitary power compensation device of the conventional art to enable prevention of isolated operation of the distributed power supply and power fluctuation compensation (flicker fluctuation compensation) to be carried out in the same manner as in the conventional art while making the equipment scale small and lessening the equipment investment by the customer.
Next, a second object of this invention is to provide the function of injecting an interharmonic into the power supplied from a distributed power supply to detect isolated operation, without the provision of a separate SVC, etc.
Another aspect of the second object of this invention is to enable, in an electrical equipment with which a plurality of the above-mentioned type of distributed power supplies are connected in the system, the detection of the system interruption, which accompanies the stoppage of the system supply, by supplying an interharmonic into the system according to each distributed power supply and without mutual interference among the distributed power supplies to prevent isolated operation of each distributed power supply without fail.
In order to achieve the above objects, a second embodiment of this invention provides: a unit which adds a supply signal for the power of interconnected operation and an injection control signal for an interharmonic and supplies the addition result to a power converter connected to a system; a unit which detects the stoppage of the system supply based on a change of an electric quantity for the interharmonic; and a unit which prevents isolated operation based on the detection.
In this case, the power and the interharmonic are supplied serially from the distributed power supply into the system via the power converter.
Based on the measurement of the interharmonic in the system, the system interruption that accompanies the stoppage of the system supply is detected from the change of an electric quantity for the interharmonic. Based on the detection, isolated operation is prevented.
The system interruption that accompanies the stoppage of the system supply can thus be detected by the distributed power supply to prevent isolated operation of the distributed power supply.
Since an interharmonic is supplied to the system to detect the system interruption that accompanies the stoppage of the system supply and the system fundamental is not modulated, ill effects, such as flicker, beating, etc. will not occur in the system as in the conventional-art active detection systems (xcex94Q system, xcex94P system) and the system quality will not be lowered.
Furthermore, since the frequency of the interharmonic is higher than the system fundamental and a change in an electric quantity for the interharmonic will thus appear in the measurements rapidly, the system interruption that accompanies the stoppage of the system supply can be detected rapidly and thus disconnection from the system can be performed rapidly.
Also, according to this invention, a distributed power supply is equipped with a unit which adds a supply signal for the power of interconnected operation and an injection control signal for an interharmonic, which is for detection of isolated operation and has a frequency that is a non-integer multiple of the system fundamental, supplies the addition result to the power converter as a drive command signal, and causes the interconnection operation power and the current of the interharmonic to be supplied serially into the system from the power converter,
a unit which detects the stoppage of the system supply from the change of the system""s electric quantity for the interharmonic based on at least either the measurement of the voltage of the interharmonic or the measurement of the current of the interharmonic, and
a unit which cuts off the power converter from the system based on the detection of stoppage of the system supply.
The power converter is thus driven by a drive command signal formed by adding a supply signal for the power of interconnected operation and an injection control signal for an interharmonic, which is for detection of isolated operation and has a frequency that is a non-integer multiple of the system fundamental. An alternating output, in which the interconnected operation power and the interharmonic are synthesized, is thus generated in the power converter and the interconnected operation power and the interharmonic current are supplied serially into the system based on this alternating output.
Also, based on at least either the measurement of the voltage of the interharmonic or the measurement of the current of the interharmonic, the system interruption that accompanies the stoppage of the system supply is detected from the change of the system""s electric quantity for the interharmonic, and based on this detection of system interruption, the power converter is cut off from the system to disconnect the distributed power supply from the system and thereby prevent its isolated operation.
The prevention of isolated operation can thus be performed, in the same manner as the above-mentioned arrangement, by a more specific arrangement.
With the power equipment of this invention, in which a plurality of the distributed power supplies are connected, the frequency of the interharmonic is differed according to each distributed power supply.
Thus for each of the distributed power supplies, the system interruption that accompanies the stoppage of the system supply can be detected and isolated operation can be prevented without mutual interference among power supplies, and the isolated operation of the plurality of distributed power supplies that are connected to the system can be prevented without fail and without the provision of a means for synchronization among the power supplies.
Furthermore, with the distributed power supply, since an interharmonic for detection of isolated operation of the distributed power supply is supplied to the system along with the interconnected operation power and via the power converter, the detection of isolated operation by means of an interharmonic can be performed by the distributed power supply itself.
A third object of this invention is to extract and detect the interharmonic of injected order at good precision and while minimizing the effects of the system components by sampling the measurement signals of the voltage and/or current of a power system by a fixed frequency sampling system in which the frequency is constant and using a Fourier transform filter calculation of the sampling data.
In order to achieve the above objects this invention provides the voltage and/or current of the power system are sampled by a fixed frequency sampling system with which the sampling frequency is constant,
the components of the interharmonic of injected order and interharmonics of non-injected orders above and below the injected order are extracted by a Fourier transform filter calculation of the sampling data,
the errors, which are based on the system voltage contained in the extracted components of the interharmonic of injected order, are interpolated from the averages of the extracted components of the interharmonics of non-injected orders, and
the errors are subtracted and eliminated from the extracted components of the interharmonic of injected order to detect the interharmonic of injected order.
The interharmonic that is injected into a power system can thus be extracted without the use of a PLL circuit, etc. and by performing sampling in an inexpensive and easy manner by fixing the sampling frequency.
With regard to the interharmonic of injected order that is extracted by the Fourier transform of the sampling data, since the errors, which are based on the system voltage and result from the non-synchronization of the system frequency and the sampling frequency are also contained in the components of interharmonic of non-injected orders above and below the injected order and these errors vary linearly in the range where the orders (frequencies) are close to each other, the errors for the injected order are determined from the average values of the components of the interharmonics of non-injected orders and are subtracted from the extracted components of the injected order.
The interharmonic of injected order can thus be detected at good precision by a fixed frequency sampling system.
For practical use, the interharmonic of injected order is preferably an interharmonic between the kth harmonic and the k+1th harmonic (where k is an integer greater than or equal to 2) and the injected frequency of the interharmonic is preferably determined from kfxc2x1mfo (where f is the frequency of the system fundamental, m is an integer of value 1, 2, . . . nxe2x88x921, n is an integer greater than or equal to 2, fo is the injection interval of the interharmonic, and fo=f/n).